Evolution of ESD Protection Diode Chip Manufacturing Process: From Planar Process to Advanced Node Integration

I.    The Central Role of ESD Protection in Integrated Circuits

• Electrostatic Discharge (ESD) protection is a fundamental pillar of integrated circuit reliability. ESD protection diodes, serving as the first and main line of on-chip defense, directly determine the survival rate of chips during manufacturing, packaging, testing, and end-use applications.
• As process nodes continue to shrink, supply voltages decrease, and gate oxide thicknesses thin, the inherent ESD robustness of chips plummets. This poses unprecedented challenges for the design and manufacturing processes of dedicated ESD devices.
• The progress in ESD diode manufacturing processes is essentially a multi-objective optimization process that balances performance, area, parasitic effects, and cost within the framework of advanced process technologies.

II.    ESD Diodes in Traditional Planar Processes

• In micrometer-scale and early sub-micrometer CMOS processes, ESD protection structures were primarily constructed using standard process devices.
• The most typical are diodes based on PN junctions, including parasitic bipolar transistors in Gate-Grounded NMOS (GGNMOS), Silicon Controlled Rectifiers (SCRs), and dedicated PN junction diodes.

• These devices require no additional masks in the process; they form P+ and N+ regions through source/drain implants, creating vertical or lateral PN junctions with the substrate or well. Their ESD performance (e.g., trigger voltage Vt1, holding voltage Vh, failure current It2) strongly depends on process parameters such as junction depth, doping concentration, silicide block (SAB) design, and contact-to-gate spacing.

For example: Using deep N-well isolation allows the construction of independent ESD protection diodes, avoiding latch-up effects. The process focus at this stage is on controlling junction characteristics by adjusting implant energy and dose, anneal conditions, and optimizing layout design rules to distribute current and prevent thermal failure.

III.    Challenges of Process Node Scaling

Entering the nanometer nodes, process evolution imposed structural limitations on ESD protection.

First    Gate oxide thickness dropped below 2nm, making its overvoltage tolerance (Vbd) far lower than ESD event voltages. This requires protection devices to respond rapidly at very low trigger voltages. Secondly   Shallow junctions and high doping gradients reduce junction thermal capacity, lowering energy dissipation capability (i.e., It2).

Finally   The replacement of aluminum with copper interconnects, which have a lower electromigration threshold, makes them more susceptible to damage under ESD high currents. Lastly, the introduction of new materials like strained silicon and high-k metal gates alters carrier mobility and thermal conductivity, affecting the transient response of ESD devices. These factors force ESD protection to shift from a "parasitic utilization" approach to a dedicated and optimized process integration path.

IV.    Dedicated ESD Process Integration Solutions for Advanced Nodes

To address these challenges, various dedicated ESD process integration technologies have emerged at 65nm and more advanced nodes:

1. Improved Silicide Block Layer and Contact Engineering: Precisely design SAB patterns to partially or fully block silicide formation in the ESD device's active area. This increases series resistance, promotes uniform current distribution, and prevents localized overheating at the junction. Simultaneously, optimize the layout and density of contact hole arrays to reduce current crowding at the contact interface.

2. Well and Implant Optimization: Introduce customized wells (e.g., deeper, more gradually doped N-well/P-well) for ESD devices through additional masks to build junctions with higher breakdown voltage and thermal capacity.

For example: In 28nm HKMG processes, using medium-voltage (MV) or high-voltage (HV) device process modules to fabricate ESD diodes. Although larger in area, they offer excellent ESD robustness due to their thicker gate oxide and deeper junctions.

3. High-Performance ESD Devices Based on BCD Processes: In chips that need to drive external loads (e.g., display drivers, power management), BCD (Bipolar-CMOS-DMOS) processes are widely used. This process naturally supports the fabrication of vertical-structure PNP/NPN transistors and diodes. The characteristics of its deep junction, low-doped epitaxial layer allow the device to withstand tens of amperes of ESD current. Breakdown characteristics and thermal failure thresholds can be precisely designed by adjusting epitaxial layer thickness and buried layer concentration.

4. ESD Design Challenges and Process Co-optimization in FinFET Processes: At 16/14nm and below FinFET nodes, the three-dimensional fin channel structure makes traditional planar ESD device design almost ineffective. FinFET's narrow fin limits current-carrying capacity, and its parasitic bipolar transistor characteristics are difficult to utilize. Current solutions mainly fall into two categories:

One category    uses planar device modules retained in the front-end-of-line (FEOL) process (e.g., I/O devices) to construct ESD protection.

The other category    integrates high-performance ESD diodes as separate chips at the packaging level, utilizing advanced silicon interposers or integrated passive device (IPD) technologies within the package. This represents "on-board" or "in-package" protection, essentially offloading some ESD process challenges from the advanced logic process.

V.    Future-Oriented Process Development Trends

The future development of ESD protection processes will present a multi-dimensional integration trend:

Process-Design Co-Optimization (PDCO): ESD performance will become a key modeling parameter in the Process Design Kit (PDK). Foundries need to provide silicon-verified ESD device IPs optimized for different interface voltage domains, and accurately characterize their performance under different ESD stress models (HBM, CDM, MM).

Exploration of New Materials and Structures: Research has begun on ESD devices based on wide-bandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN). They offer higher thermal conductivity and critical breakdown field strength, promising use in extreme environments. Furthermore, the mechanisms of ESD devices under Gate-All-Around (GAA) structures like nanowires and nanosheets are also frontier research topics.

System-Level Protection and Heterogeneous Integration: With the proliferation of Chiplet and 3D integration technologies, ESD protection will increasingly shift from transistor-level to system-level considerations. Fabricating thin-film ESD diodes on substrate interposers or redistribution layers (RDL) and heterogeneously integrating them with core chips will become an important path to balance performance, cost, and process compatibility.

Conclusion

The chip manufacturing process for ESD protection diodes has evolved from early parasitic utilization based on standard CMOS processes to a critical technology module requiring deep customization and co-optimization at advanced nodes. Its progress is reflected not only in the precise control of detailed processes like well engineering, implant schemes, and silicide control, but also in the overall design paradigm shift from planar to three-dimensional structures and from single-chip to system-in-package. The robustness of ESD processes remains the invisible pillar supporting the reliability of the integrated circuit industry.

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